Integration and power saving of semiconductor devices (in particular, silicon devices) have been promoted by miniaturization (scaling law: Moore's law) of devices at a pace of quadrupling in three years in terms of development. A gate length of a metal oxide semiconductor field effect transistor (MOSFET) has become 20 nm or less in recent years, and substantial rise in a lithography process cost (a device price and a mask set price) and a physical limit (an operating limit and a variation limit) of a device size have necessitated improved device performance by an approach different from the conventional scaling law.
A rewritable programmable logic device called a field programmable gate array (FPGA) positioned between a gate array and a standard cell has been developed in recent years. The FPGA allows a customer himself/herself to perform any circuit configuration after the chip is manufactured. The FPGA includes a variable-resistance element inside a multilayer wiring layer and allows a customer himself/herself to form an arbitrary electrical connection in wiring. Use of a semiconductor device equipped with such an FPGA enables improvement of flexibility in circuit design.
Memories using a variable-resistance element include a magneto-resistive random access memory (MRAM), a phase change RAM (PRAM), a resistance random access memory (ReRAM), and a conductive bridging random access memory (CBRAM: RAM that uses conductive path formed by ions of solid electrolyte).
The ReRAM uses a characteristic that a resistance value varies by externally applied voltage and current, either in an ON-state in which a conductive path is formed inside a variable-resistance film forming a variable-resistance element, or, conversely, in an OFF-state in which the conductive path formed inside the variable-resistance film is eliminated. Accordingly, the ReRAM cell uses a structure including a variable-resistance film composed of a metallic oxide, sandwiched between two electrodes. For example, an ON-state is generated by applying an electric field to the variable-resistance film in such a way as to form a filament inside the variable-resistance film or to form a conductive path between the two electrodes. Then, on the other hand, an OFF-state is generated by applying an electric field in an inverse direction to the variable-resistance film in such a way as to eliminate the filament or to eliminate the conductive path formed between the two electrodes. By reversing the direction of the electric field applied to the variable-resistance film, switching between the ON-state and the OFF-state, is performed, the states having greatly different resistance values between the two electrodes. Data are stored by using current flowing through the variable-resistance element, being different depending on a difference in the resistance value between the aforementioned ON-state and OFF-state. When data are written, depending on data to be stored, a voltage value, a current value, and a pulse width that cause transition from the OFF-state to the ON-state or transition from the ON-state to the OFF-state is selected, and generation or elimination of the filament for data storage, or formation or elimination of the conductive path is performed.
Non-Patent Literature 1 (NPL1) discloses a variable-resistance element that is highly likely to improve flexibility of a “circuit” used in a “memory cell” configuration in an ReRAM, as a type of a variable-resistance element used in an ReRAM configuration. The variable-resistance element is a nonvolatile switching element that reversibly changes a resistance value between electrodes sandwiching the variable-resistance film, by using metal ion movement in an ion conductor, and “precipitation of metal by reduction of a metal ion” and “generation of a metal ion by oxidation of metal” by an electrochemical reaction, and performs switching. The nonvolatile switching element disclosed in NPL1 is composed of a “solid electrolyte” composed of an ion conductor, and a “first electrode” and a “second electrode” provided in contact with two surfaces of the “solid electrolyte,” respectively. A “first metal” constituting the “first electrode” and a “second metal” constituting the “second electrode,” constituting the nonvolatile switching element, have different values of standard Gibbs energy of formation ΔG in a process of oxidizing a metal and generating a metal ion.
In the nonvolatile switching element disclosed in NPL1, the “first metal” constituting the “first electrode” and the “second metal” constituting the “second electrode” are respectively selected as follows.
When “bias voltage” causing transition from an OFF-state to an ON-state is applied between the “first electrode” and the “second electrode,” a metal capable of supplying a metal ion to a “solid electrolyte,” by the metal being oxidized by an electrochemical reaction induced by the applied “bias voltage” and the metal ion being generated, is employed as the “first metal” constituting the “first electrode” at an interface between the “first electrode” and the “solid electrolyte.”
When “bias voltage” causing transition from an ON-state to an OFF-state is applied between the “first electrode” and the “second electrode” and the “first metal” is precipitated on a surface of the “second electrode,” while, with regard to the “first metal” precipitated on the surface of the “second electrode,” the metal is oxidized by an electrochemical reaction induced by the applied “bias voltage,” generates a metal ion, and dissolves into the “solid electrolyte” as the metal ion, with regard to the “second metal” constituting the “second electrode,” a metal that, depending on the applied “bias voltage,” may not induce a process of the metal being oxidized and a metal ion being generated, is employed.
A switching operation in a metal bridge type variable-resistance element achieving an ON-state and an OFF-state by “formation of a metal bridge structure” and “dissolution of a metal bridge structure” will be briefly described.
In a transition process (setting process) from an OFF-state to an ON-state, when the second electrode is grounded and the first electrode is applied with positive voltage, a metal in the first electrode turns to a metal ion and dissolves into a solid electrolyte at an interface between the first electrode and the solid electrolyte. On the other hand, on the second electrode side, by using an electron supplied by the second electrode, a metal ion in the solid electrolyte turns to a metal in the solid electrolyte and is precipitated. A metal bridge structure is formed by metals precipitated in the solid electrolyte and a metal bridge connecting the first electrode and the second electrode is finally formed. A switch goes into an ON-state by electrically connecting the first electrode and the second electrode by the metal bridge.
On the other hand, in a transition process (resetting process) from an ON-state to an OFF-state, when the second electrode is grounded and the first electrode is applied with negative voltage with respect to an ON-state switch, a metal constituting a metal bridge turns to a metal ion and dissolves into the solid electrolyte. As the dissolution progresses, part of a “metal bridge structure” constituting the metal bridge breaks. When the metal bridge connecting the first electrode and the second electrode finally breaks, the electrical connection breaks, and the switch goes into an OFF-state.
Note that, as the dissolution of metals progresses, electrical characteristics change in a stage before the electrical connection completely breaks in such a way that resistance between the first electrode and the second electrode increases due to narrowing of the “metal bridge structure” constituting a conduction path, and also, inter-electrode capacity changes due to dissolved metal ions being reduced and precipitated as metals at the interface between the first electrode and the solid electrolyte, leading to decreased concentration of metal ions contained in the “solid electrolyte” and change in a relative dielectric constant, and then the electrical connection finally breaks.
Further, when the second electrode in the metal bridge type variable-resistance element caused to transition (reset) to an OFF-state is grounded and the first electrode is applied with positive voltage again, a transition process (setting process) from the OFF-state to an ON-state progresses. In other words, in the metal bridge type variable-resistance element, a transition process (setting process) from an OFF-state to an ON-state and a transition process (resetting process) from an ON-state to an OFF-state can be performed reversibly.
Further, NPL1 discloses a configuration and a switching operation of a two-terminal-type switching element including two electrodes arranged through an ion conductor and controlling a conduction state between the two electrodes.
(Definition of Polarity of Variable-resistance Element)
Regardless of the aforementioned operating principle, an operating characteristic of a variable-resistance element applicable to the present invention can be classified into a unipolar type performing a variable-resistance operation, based on an applied voltage level, and a bipolar type performing a variable-resistance operation, based on an applied voltage level and a voltage polarity. It is preferable to use a bipolar-type variable-resistance element in the present invention.
<Description of Solid-electrolyte-layer-type Variable-resistance Element>
As an example of the aforementioned bipolar-type variable-resistance element, NPL1 discloses a switching element using metal ion movement and an electrochemical reaction in a solid electrolyte layer (a solid in which an ion is able to move freely by application of an electric field and the like). The switching element disclosed in NPL1 is composed of three layers of a solid electrolyte layer, and a first electrode and a second electrode that are arranged to face one another abutting the solid electrolyte layer at one side and another side opposite to the one side, respectively. The first electrode plays a role of supplying a metal ion to the solid electrolyte layer. The second electrode does not supply a metal ion.
An operation of the switching element will be briefly described below.
When the first electrode is grounded and the second electrode is applied with negative voltage, a metal in the first electrode turns to a metal ion and dissolves into the solid electrolyte layer. Then, the metal ion in the solid electrolyte layer turns to a metal and is precipitated in the solid electrolyte layer. By metals precipitated in the solid electrolyte layer, a metal bridge connecting the first electrode and the second electrode is formed. By the first electrode and the second electrode electrically connected by the metal bridge, the switching element goes into an ON-state.
On the other hand, when the first electrode is grounded and the second electrode is applied with positive voltage in the aforementioned ON-state, part of the metal bridge breaks. Consequently, the electrical connection between the first electrode and the second electrode breaks, and the switching element goes into an OFF-state. Note that, the electrical characteristics of the electrical connection change in a stage before the electrical connection completely breaks, in such a way that resistance between the first electrode and the second electrode increases, capacity between the first electrode and the second electrode changes, and the like, and then the electrical connection finally breaks.
Further, in order to change from the aforementioned OFF-state to an ON-state, the first electrode may be grounded and the second electrode may be applied with negative voltage, again.
As a switching element by a solid-electrolyte-layer-type variable-resistance element, NPL1 discloses a configuration and an operation of a two-terminal-type switching element including first and second electrodes arranged through a solid electrolyte layer and controlling a conduction state therebetween.
A switching element by such a solid-electrolyte-layer-type variable-resistance element features a smaller size and less ON-resistance compared with a semiconductor switch such as a MOSFET. Accordingly, the switching element is considered promising for application to a programmable logic device.
Further, in the switching element, a conduction state (ON or OFF) thereof is maintained intact even when applied voltage is turned off. Accordingly, application to a nonvolatile memory element may also be considered. For example, with a memory cell including one selection element, such as a transistor, and one switching element, as a basic unit, a plurality of the memory cells are arranged in a longitudinal direction and a transverse direction, respectively. Such an arrangement enables selection of any memory cell with a word line and a bit line, out of the plurality of memory cells. Then, a nonvolatile memory capable of sensing a conduction state of the switching element in the selected memory cell and reading which information of information “1” or “0” is stored, from an ON-state or OFF-state of the switching element, can be provided.
With regard to a nonvolatile variable-resistance element, Patent Literature 1 (PTL1) discloses a configuration provided with a first electrode, a second electrode, a variable resistor connected to both of the first electrode and the second electrode, and a control electrode (third electrode) connected to the variable resistor through a dielectric layer, the dielectric layer being in contact with a side surface of a second variable resistor.
Patent Literature 2 (PTL2) relates to a memory circuit holding wiring connection information and logic information, and proposes connecting a first variable-resistance element, a second variable-resistance element, and a first switching element in series between a first power source and a second power source.